There is a continuing need to monitor the level of liquid in an in-ground liquid storage tank. Such in ground liquid storage tanks are typically employed for storage of liquid from the time of delivery until use. Such storage tanks are also used for temporary storage of liquid at a wholesaler's location from the time of receipt until it is removed for delivery to a retailer or at a retail location from the time of delivery until sale to the customer. Such in-ground storage tanks are typically formed of cylinders having a substantially vertical fill pipe connecting the tank interior to a surface tank inlet permitting the tank to be filled. It is typical to include an outlet at the bottom of the tank whereby liquid may be withdrawn from the tank for use or delivery to a customer.
It is necessary to periodically monitor the liquid level in such in-ground storage tanks for a number of reasons. Firstly, in the case of tanks where the outlet of liquid is unmetered there exists a need to determine the liquid level within the tank. In addition, even if the outlet of the tank is metered it is necessary to periodically check the liquid level within the tank in order to ensure the accuracy of the inflow and outflow metering. In addition, it is advisable to periodically independently determine the liquid level within a tank in order to determine whether or not the tank is leaking. Even with metered inflows and outflows, such liquid level measurement is required for leak detection. Such leak detection is particularly desirable in the case in which the liquid stored in the storage tank is some hazardous or potentially hazardous material. Often there are environmental regulations requiring the owners or users of such in-ground liquid storage tanks to periodically test for leaks. As a consequence, it is desirable test the liquid level within such tanks.
Heretofore two primary systems have been employed for determining the liquid level within the tank. The first method is a purely manual method. This method requires a stick having a length somewhat greater than the length from the top of the fill pipe to the bottom of the tank at that location. The operator lowers the stick into the tank until it reaches the bottom. The stick is then withdrawn and the level of the liquid is observed against a scaled rule on the stick. This liquid depth is then converted manually into the corresponding volume of liquid within the tank based upon the known tank parameters such as diameter, length and the length of the fill pipe. This technique requires a relatively large labor cost and, due to the manual calculations required, provides a relatively low level of accuracy.
The second known technique for measuring the liquid level for such in-ground storage tanks involves the use of an in-tank electronic measuring apparatus. Such in-tank liquid level measuring apparatus typically employ an ultrasonic ranging technique. The ultrasonic ranging may occur within the liquid in the tank from the bottom of the tank to the liquid surface or from the top of the tank down to the liquid surface. Because it is known that the speed of sound changes with ambient temperature, many such systems employ ultrasonic reflection targets at a fixed distance form the transmit/receive transducer. The return echo from this fixed reference is employed in order to calibrate the return echo from the liquid level.
These in-tank liquid level sensors typically are very expensive and require a large amount of time to mount in an existing tank. As such this is a very expensive solution for retrofit to existing tanks. In addition, it is very expensive and extremely cumbersome to service or to modify such in-tank liquid level monitors. Accordingly there is a need in the art for a inexpensive, easy to install liquid level monitoring device.